This invention relates to polyolefin films, and more particularly to the improvement of heat-seal strength through improvement of inter-layer bonding properties of such films.
The polymers normally employed in the preparation of biaxially-oriented polypropylene films are isotactic polymers such as isotactic polypropylene, although on some occasions the use of syndiotactic polymers has been proposed. Isotactic polypropylene is one of a number of crystalline polymers which can be characterized in terms of the stereoregularity of the polymer chain. Various stereo-specific structural relationships denominated primarily in terms of syndiotacticity and isotacticity may be involved in the formation of stereoregular polymers for various monomers.
Isotactic polypropylene is conventionally used in the production of relatively thin films in which the polypropylene is heated and then extruded through dies and subjected to biaxial orientation by stressing the film in both a longitudinal direction (referred to as the machine direction) and in a transverse or lateral direction sometimes referred to as the xe2x80x9ctenterxe2x80x9d direction. The structure of isotactic polypropylene is characterized in terms of the methyl group attached to the tertiary carbon atoms of the successive propylene monomer units lying on the same side of the main chain of the polymer. That is, the methyl groups are characterized as being all above or below the polymer chain. Isotactic polypropylene can be illustrated by the following chemical formula: 
[Bolding for Emphasis Only]
Another way of describing the structure is through the use of NMR. Bovey""s NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each xe2x80x9cmxe2x80x9d representing a xe2x80x9cmesoxe2x80x9d dyad, or successive methyl groups on the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
The isotactic polymers normally employed in the preparation of biaxially-oriented polypropylene films are usually those prepared through the use of conventional Ziegler-Natta catalysts of the type disclosed, for example, in U.S. Pat. Nos. 4,298,718 and 4,544,717, both to Myer et al. Thus, U.S. Pat. No. 5,573,723 to Peiffer et al discloses a process for producing biaxially-oriented polypropylene film based on an isotactic polypropylene homopolymer or propylene-ethylene copolymers. Other copolymers of propylene and alpha-olefins having from 4-8 carbon atoms also may be employed in the Peiffer process.
Catalysts employed in the polymerization of alpha-olefins may be characterized as supported catalysts or unsupported catalysts, sometimes referred to as homogeneous catalysts. Traditional supported catalysts are the so-called xe2x80x9cconventionalxe2x80x9d Ziegler-Natta catalysts, such as titanium tetrachloride supported on an active magnesium dichloride as disclosed, for example, in the aforementioned patents to Myer et al. A supported catalyst component, as disclosed in the Myer ""718 patent, includes titanium tetrachloride supported on an xe2x80x9cactivexe2x80x9d anhydrous magnesium dihalide, such as magnesium dichloride or magnesium dibromide. The supported catalyst component in Myer ""718 is employed in conjunction with a co-catalyst such as an alkylaluminum compound, for example, triethylaluminum (TEAL). The Myer ""717 patent discloses a similar compound which may also incorporate an electron donor compound which may take the form of various amines, phosphenes, esters, aldehydes, and alcohols. Metallocene catalysts are often employed as unsupported or homogeneous catalysts, although, as described below, they also may be employed in supported catalyst components.
Alternative types of catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403. These patents disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene.
The present invention relates to multi-layer polyolefin film. The film includes a core layer formed of an ethylene-propylene copolymer where the ethylene is present in an amount of no more than about 1 weight percent, preferably between about 0.1 weight percent and about 0.7 weight percent, and most preferably between about 0.3 weight percent and about 0.5 weight percent. The film has a surface layer contiguous to and bonded with the core layer, where the core layer has a thickness greater than the thickness of the surface layer. The surface layer is a thermoplastic polymer capable of forming an effective heat seal with a corresponding thermoplastic polymer upon heating to an elevated temperature and compression. The ethylene-propylene copolymer forming the core layer has an isotactic structure and contains ethylene in an amount which is effective to provide an inter-layer bond strength with the surface layer which is at least about 15 percent greater than the inter-layer bond strength between the surface layer and a film formed of isotactic polypropylene homopolymer. The present invention further relates to a method for producing such a multi-layer film.